There are various advantages which make it desirable or necessary to light the inside of a swimming pool or other large water tanks. To do this, underwater lamps are installed at different points on the walls of the tank.
The lamp consists of a bulb with or without convex crowns, powered by electricity and installed inside a surrounding shell. This shell is placed inside a recess in the wall and its outside side is covered by a screen of glass or other sufficiently transparent material. When the lamp is on, it projects a beam of light rays inside the water, the symmetry axis of which is provided by the characteristics of the bulb and the reflector inside the shell. The nature of water means that in very short stretches of rays, the beam becomes very diffused, but always maintains the symmetry axis.
At present, these lamps are installed so that the axis of the beam is parallel to the horizontal, or slanted downwards at an angle of between 4.degree. and 8.degree..
This downward slant is desirable in many private swimming pools to avoid waves on the water surface from refracting light outside the pool, which would light the surrounding area, causing an undesired effect.
Also, once certain prefabricated pools are installed, their section profile alters from rectangular to slightly trapezoidal with the short side of the trapezoid at the base. The reason for this is the pressure of earth, which is greater at the deeper parts of the walls, which means that these move slightly towards the inside of the pool. On these walls, lamps with a downward axis have to be installed, as a horizontal axis would light upwards.
At the present time lamps installed with a horizontal axis cannot be installed with a sloping axis. Stores have to stock both types. The fitter has to choose one type or another when carrying out the installation. If the direction of the axis has to be changed after installation, the whole lamp has to be changed.
Before making the recess in the wall, it must be known from which direction the cables will reach the lamp, and depending on this direction, the elbow inlet duct of cables to the recess is placed accordingly. Using current techniques, the elbow duct is fixed to the lamp in its final position at the set direction. In the event that it is later seen that this direction is incorrect because the labourer made the recess for the lamp, or the cable channel at a slightly different position, which in reality occurs more often than installers would like, the lamp has to be discarded and a new one installed.